The present invention relates to a method of manufacturing spherical alumina particles according to the oil-drop method. In particular, it relates to a method which comprises preparing alumina hydrosol, a precursor of said spherical alumina particles, by using gibbsite as the starting material, and obtaining spherical alumina particles from said alumina hydrosol by means of the known oil-drop method.
Alumina hydrosol transformable into spherical alumina particles by the oil-drop method has long been prepared by using a method of digesting metallic aluminum witnh an aqueous hydrochloric acid solution and/or an aqueous aluminum chloride solution. However, this method is defective in that the use of said metallic aluminum results in expensive spherical alumina particles because the metallic aluminum is itself expensive. Accordingly, in order to reduce the high cost of the spherical alumina particles there has been proposed a method of preparing the alumina hydrosol transformable into spherical alumina particles by the oil-drop method from gibbsite in Japanese Patent Publication No. 31118/1978, Japanese Laid Open Patent Application No. 60022/1980.
The method disclosed in Japanese Patent Publication No. 31118/1978 comprises preparing an aluminum chlorohydroxide solution by etching gibbsite powders with hydrochloric acid at a temperature ranging from 110.degree. to 150.degree. C., neutralizing the resulting solution with ammonia so that the solution has a pH ranging from 2.7 to 3.5, thereafter allowing ammonium chloride to crystallize at a temperature ranging from 0.degree. to 25.degree. C., and separating the resulting crystals together with other solid impurities from the solution. This method is surely advantageous in that alumina hydrosol can be prepared without using metallic aluminum but is troublesome in that the by-product ammonium chloride must be crystallized for purposes of removal.
On the other hand, Japanese Laid Open Patent Application No. 60022/1980 teaches a method which comprises preparing a basic aluminum chloride solution having an aluminum concentration in the range of from 7 to 12 wt.% and a weight ratio of aluminum to chloride (which will be referred to as Al/Cl ratio hereinafter) in the range of from 0.3 to 0.8 by reacting gibbsite with an aqueous hydrochloric acid solution at an elevated temperature and then reacting the resulting solution with metallic aluminum at an elevated temperature, thereby obtaining alumina hydrosol having an aluminum concentration in the range of from 9 to 15 wt.% and an Al/Cl ratio in the range of from 0.8 to 1.5. According to this method, wherein ammonia is not employed as in the aforesaid method of Japanese Patent Publication No. 31118/1978, there is no fear of ammonium chloride being by-produced. However, this method leaves room for improvement on the point of reducing the cost of the spherical alumina particles because the prior art method permits gibbsite to replace only about 50% of the overall amount of aluminum contained in the alumina hydrosol.
The present inventors have conducted out a series of studies on a method of preparing alumina hydrosol suitable for use in the oil-drop method from gibbsite while taking account of the above-mentioned two known techniques and consequently have discovered that, when a basic aluminum chloride solution having a specific range of aluminum concentration and Al/Cl ratio is prepared by digesting gibbsite with an aqueous hydrochloric acid solution at an elevated temperature and an elevated pressure, and the resulting basic aluminum chloride solution is neutralized with an ammonia water to thereby obtain alumina hydrosol having an aluminum concentration as well as an Al/Cl ratio suitable for the oil-drop method, the by-product ammonium chloride co-existing in the alumina hydrosol does not detrimentally affect the physical properties of the spherical alumina to be produced from this hydrosol by the oil-drop method. Instead, the ammonium chloride by-product functions to increase the viscosity of a mixture of alumina hydrosol and gelling agent as well as increase the gelation speed of said mixture in an oil bath, and as a result of this increased gelation speed, gelation of the hydrosol can be effected at a practical speed even when the greater part of the gelling agent is replaced by urea.